Initial release

Changes:

• marshmallow updated to 3+
• improved filters:
  • support for nested attributes filtering (dot-separated), for example filter=[{"name": "owner_id.group_id.name", "op": "ilike", "val": "spam%"}] - here we have some Thing with relationship on owner_id to a User with relationship on group_id to a Group with an attribute name
  • support for custom filters
• improved sorting:
  • the values for sorting are now serialized/deserialized
  • the same deep sorting for nested attributes same as with filtering
  • support for custom sorting
• Plugin system (reference)